It is highly documented that military personnel exposed to blasts from firearms, explosions and other high level peak noises are at high risk for hearing loss. It has been estimated that 68% of deployed soldiers return with ear damage, and 100% do not protect both ears during combat. (Saunders and Griest, “Hearing loss in veterans and the need for hearing loss prevention programs” NHCA Meeting, Portland Oreg., Feb. 23, 2008). In 2007, it was estimated that the cost of hearing aids for soldiers was $141.3 million and the cost for clinical services (not including hearing aids) was $147.1 million. (Saunders and Griest, “Hearing loss in veterans and the need for hearing loss prevention programs” NHCA Meeting, Portland Oreg., Feb. 23, 2008).
Sound pressure levels (SPLs) over 160 dB that occur over periods as short as even a few milliseconds are sufficient to cause damage to the unprotected ear. Exposure to the peak noises caused by gunfire or other explosions adds hearing loss to the long list of risks and dangers encountered by soldiers on the battle field. One means to prevent hearing loss is to wear a noise attenuating device such as ear plugs or earmuffs. U.S. Pat. No. 5,203,352 issued to Gardner presents high-attenuation foam earplugs which may provide up to 40 dB of attenuation when properly inserted. Accordingly, the Gardner earplugs will reduce hazardous external peak SPLs of 160 to 190 dB to safer levels of 120 to 150 dB, respectively, within the ear canal of the wearer.
The Gardner and other similar earplugs will attenuate up to 40 dB of noise, but the attenuation level is independent of the level external sound. In other words, all external noises will be attenuated the same amount whether the sounds are extremely loud or very soft. Thus, softer sounds that would otherwise be audible without the use of earplugs may become inaudible or become so soft that they go unnoticed. For many work environments the perception of soft sounds is vital to the task at hand or the safety of the workers. For example, a soldier wearing earplugs as described attenuating a constant 40 dB of noise may fail to hear an enemy quietly approaching or fail to perceive communications from fellow soldiers. Likewise, a construction worker wearing such earplugs may receive adequate protection from high level construction sounds, but fail to hear a distant coworker's emergency call for help.
Many earplugs, like those described by Gardner, for example, may distort the reception of normal sound. The earplugs attenuate higher frequency sounds at a higher level than lower frequency sounds making it difficult for the wearer to hear or understand speech and other important sounds. High-audibility earplugs such as those described by U.S. Pat. No. 4,807,612 issued to Carlson and used in the ER-9®, the ER-15®, and the ER-25® series Musicians Earplugs® produced by Etymotic Research, Inc., and having nominal 9 dB, 15 dB, and 25 dB attenuation respectively, and as described in U.S. Pat. No. 5,113,967 issued to Killion et al., and used in products such as the ER-20® series HiFi earplugs produced by Etymotic Research, Inc.®, all produce relatively uniform attenuation across audible frequency ranges and a low enough attenuation such that speech and music remain highly audible to the wearer. While the high audibility of these earplugs allows the wearer to hear softer noises, they may not provide adequate protection for extremely high level sounds. Moreover, even the lowest-attenuation ER-9 earplug may produce unacceptable attenuation for a soldier who needs to have the best chance of hearing a quietly approaching enemy. Moreover 9 dB attenuation is insufficient to protect the soldier's ears from many high-energy noise levels that occur during battle.
An earplug with sound level dependent attenuation is described in U.S. Pat. No. 4,924,502 issued to Allen, et al. and U.S. Pat. No. 5,936,208 issued to Hamery and is embodied in a product sold by AEARO Technologies under the trade name Combat Arms Earplug. The Combat Arms Earplug introduces less noise attenuation for external SPLs below 110 dB than for external SPLs above 110 dB, but does not provide a constant attenuation across all frequencies. For example, where the external SPL is below 110 dB the Combat Arms Earplug provides around 5 dB of attenuation at very low frequencies and up to 23 dB attenuation at higher frequencies. Where the external SPL is above 110 dB, the attenuation provided increases by approximately 0.3 dB for each 1 dB increase of external sound until a maximum level of attenuation is achieved. For example a 9 dB change for 110-150 input gives 9/40=0.23 dB/dB. For the 150-190 range, the attenuation may increase another 12 dB in that 40 dB range, or 0.3 dB/dB. The Combat Arms Earplug provides a passageway that allows partially unobstructed travel of sound from the exterior into the ear canal with a relatively low level of attenuation at low sound pressure levels. A sharp obstruction located within the passageway of the Combat Arms Earplug causes the flow of sound within the earplug to become turbulent above 110 dB. This introduction of turbulent flow impedes the flow of the sound into the ear, thereby establishing greater attenuation. While the aforementioned earplugs may provide non-linear pattern attenuation, they do not provide enough audibility for low-level sounds: The 15-23 dB attenuation they provide between 1 kHz and 3 kHz (Berger and Hamery “Empirical evaluation using impulse noise of the level-dependency of various passive earplug designs;” Presentation at Acoustics Paris 2008 J. Acoust Soc. Am. 123 (5, Pt. 2), p. 3528) is apparently too much to provide good situation awareness for the soldier. As a result, “Many troops say they don't use hearing protection on missions because they feel it affects their situational awareness, ability to do their jobs and complete their missions” according to Dr. Joseph Brennan in an Army Times staff report “Troops reject ear protection in Afghanistan” Posted: Wednesday Aug. 26, 2009 5:29:32 EDT. Other estimates are that 68% of deployed soldiers return with ear damage, and the majority refuse to wear earplugs, costing some $288 million each year in auditory rehabilitation (Saunders and Griest, “Hearing loss in veterans and the need for hearing loss prevention programs” NHCA Meeting, Portland Oreg., Feb. 23, 2008).
It seems clear from the refusal of soldiers to wear existing hearing protection that no existing devices provide the combination of situational awareness (especially for quiet sounds) and blast protection that is needed.
Electronic hearing protection would appear to be a better solution, and indeed has also been known for many years. A popular form is the “Walker Game Ear” device. This and similar forms typically provide some gain for quiet sounds under the assumption that the typical hunter purchaser already has some hearing loss so that a combination hearing aid and hearing protection would be beneficial. They all have in common that they use a tiny hearing aid receiver whose maximum output of 110-125 dB is further limited by a limited power available from the output amplifier in the hearing aid. Thus the peak pressure from a firearm of typically or 170 dB is reduced to a snap of less than 130 dB, which is safe for the 1 or 2 milliseconds of the pulse duration. According to Berger, firearms, howitzers and roadside explosions typically fall in the range of 150 to 180 dB SPL. This electronic approach is well known to the applicant, since the “K-AMP” hearing aid (which was designed by Etymotic Research, described in U.S. Pat. No. 5,131,046, and manufactured by nearly every hearing aid company in the 1990s) was used for hearing protection by many including one of its own employees while hunting. The K-AMP Hearing Aid had the advantage that it provided normal localization for the hunter and near-normal “situational awareness,” but a drawback was that it had to be built into a custom earmold shell which typically required 2-3 visits to a licensed hearing aid dispenser.
The problem with all present devices is that they do not provide a low-cost, ready-to-wear, high-fidelity device with a 16 kHz bandwidth, suitable for the normal-hearing soldier. The lack of understanding that this is even possible is indicated perhaps by the fact that in many pages of Army “SBIR” requests for proposals perused by the applicant, none of them describe the type of device that the applicant has developed, or apparently even imagined that it was possible.
Additionally, when switching between gain level settings of an electronic earplug, there is a need for providing audible switch indications while introducing electronic control of the gain using transistor switching.